Accurate placement and verification of treatment fields remain as principal problems in the delivery of radiation therapy. Measurements of patient setup and field positioning errors have been reported by many authors. For example, many published reports document positioning errors having mean deviations of the order of 5 to 8 mm, wherein a significant percentage of positioning errors occur in excess of 15 mm. One example of such reports can be found in an article entitled "Quality Assurance in Radiation Therapy: Physics Efforts" by Svensson, G. K., Int. J. Rad. Onc. Biol. Phys., Vol. 10, Sup 1, 23-29, 1983.
As is well known, such positioning errors lead to a decrease in Tumor Control Probability (TCP) which can be as large as 20 percent (20%). One example of such results can be found in an article entitled "Uncertainty Analysis of Field Placement Error Measurements Using Digital Portal and Simulation Image Correlations" by McParland, B. J., Med. Phys., 20(3), 679-685, May/June 1993.
Many groups have attempted to address this issue by using "port films" or real time portal imaging technology. One example of such attempts is described in an article entitled "Automatic On-line Inspection of Patient Set-up in Radiation Therapy Using Digital Portal Images" by Gilhuijs, K. G. A. and van Herk, M., Med. Phys., 20(3), May/June 1993. See also U.S. Pat. No. 5,138,647 entitled "Portal Imaging Device" issued Aug. 11, 1992 to Nguyen, J. and Yu, C. X. and assigned to Siemens Medical Laboratories, Inc.
Presently, routine clinical implementation of the above-described techniques suffers from drawbacks in that they: (a) are labor intensive; (b) require human judgment; and (c) require delivery of radiation before patient/field position can be determined. In addition, most analysis is done off-line after patient treatment has been completed.
At present, no commercial system exists which can rapidly, reliably, remotely and accurately measure the orientation/position of a patient in Cartesian space by performing the following tasks: (a) resolve target position on a patient to better than 1 mm absolute in Cartesian space and determine patient rotation about any of the three principal axes; (b) provide sufficient reproducibility so that an operator/technologist can reposition targets on the patient with a reproducibility of better than 2 mm, from day to day; (c) warn the operator/technologist if the patient, i.e., the targets, are not in the correct position before treatment; (d) report patient position with respect to an initial, i.e., reference, patient setup or with respect to a setup for a particular treatment; and (e) provide a method for quality assurance of a linac ODI, laser, light field position and digital couch. Thus, there is a need in the art for a system for accurate patient setup and day-to-day patient position verification which provides these capabilities.